A technique has been devised for estimating the fraction of the total polymerase present in a subcellular fraction of E. coli that is present as a complex. The technique involves determination of the ATP-stimulated DNA synthesis with native DNA and the activity with activated DNA. Polymerase in DNA synthesizing complex is active with native DNA and is stimulated by ATP while free DNA polymerase I has little activity with native DNA with or without ATP. Activated DNA measures the total polymerase activity. The ratio (R) of the ATP-stimulated activity with native DNA to the total activity with activated DNA is thus a measure of the fraction of polymerase in complex and can be calculated from the formula C/T = RM-Rp/Rc-Rp, where C/T is the fraction of total polymerase in complex and RM, Rp and Rc are the activity ratios for the mixture, free polymerase and complexed polymerase, respectively. In a typical preparation 33% of the polymerase in the starting homogenate was in the form of complex. Attempts to further purify E have continued utilizing differential centrifugation, affinity chromatographpy and detergent solubilization.